Moving picture real-time communications terminal, control method for moving picture real-time communications terminal, and control program for moving picture real-time communications terminal

ABSTRACT

The moving picture real-time communications terminal comprises: an input unit which inputs an RGB video image signal; a conversion unit which converts the RGB video image signal into a YUV video image signal including Y, U and V video image signals; a compression and encoding unit which digitally converts the YUV video image signal outputted from the conversion unit into moving picture data, and compresses and encodes the moving picture data; a communications unit which sends and receives the moving picture data compressed and encoded by the compression and encoding unit, in real time, to and from a communications terminal of another party connected through a network; a bandwidth estimation unit which estimates change in a bandwidth of the network; a switching circuit which connects and disconnects output paths of the U and V video image signals from the conversion unit to the compression and encoding unit; and a switching control unit which disconnects the output paths of the U and V video image signals by operating the switching circuit, in response to the change in the bandwidth estimated by the bandwidth estimation unit, wherein the compression and encoding unit increases a bit rate of compression and encoding of the Y signal, in accordance with disconnection of the output paths of the U and V video image signals.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to moving-picture communication, and moreparticularly to moving-picture communication through a network ofvarying bandwidth, and to technology which ensures prescribed imagequality by switching between color moving pictures and black and whitemoving pictures.

2. Description of the Related Art

In moving-picture communication, various technologies have beendeveloped for dynamically adjusting the transmitted data volume inaccordance with the bandwidth of the network environment. For example,Japanese Patent Application Publication No. 2002-77857 discloses thatre-sequencing and/or selection of the data of respective elements inmultimedia data is carried out on the basis of network informationrelating to the bandwidth, and the corresponding results are storedtemporarily in a buffer. The transmission engine processes themultimedia data in accordance with these results, and, for example,omits frames from the video image data, when sending the data to theuser terminal.

When video images are exchanged mutually in real time between terminalswhich are connected through a communications network, such as that in avideo telephone and a video conferencing system, there may be a problemin that the video image data has missing frames or the frame rate islowered if image quality is prioritized (in other words, maintaining ahigh compressed bit rate in each frame unit). For example, in a videotelephone for sign language as used by persons with hearingdifficulties, the facial expression and the manual gestures of the othercommunicating party are important, and if the observer sees a videoimage of the other party which does not display smooth movement due tomissing frames, then it is not possible to recognize the changes in thefacial expression, the mouth movements, and the manual gestures, of theother party, and hence it is difficult to understand the contents of thesign language.

SUMMARY OF THE INVENTION

The present invention has been contrived in view of the foregoingcircumstances, an object thereof being to improve image quality, withoutlosing smoothness in the movement of the video image, even if thetransmission bandwidth of the network is reduced.

In order to attain the aforementioned object, the present invention isdirected to a moving picture real-time communications terminal,comprising: an input unit which inputs an RGB video image signal; aconversion unit which converts the RGB video image signal into a YUVvideo image signal including Y, U and V video image signals; acompression and encoding unit which digitally converts the YUV videoimage signal outputted from the conversion unit into moving picturedata, and compresses and encodes the moving picture data; acommunications unit which sends and receives the moving picture datacompressed and encoded by the compression and encoding unit, in realtime, to and from a communications terminal of another party connectedthrough a network; a bandwidth estimation unit which estimates change ina bandwidth of the network; a switching circuit which connects anddisconnects output paths of the U and V video image signals from theconversion unit to the compression and encoding unit; and a switchingcontrol unit which disconnects the output paths of the U and V videoimage signals by operating the switching circuit, in response to thechange in the bandwidth estimated by the bandwidth estimation unit,wherein the compression and encoding unit increases a bit rate ofcompression and encoding of the Y signal, in accordance withdisconnection of the output paths of the U and V video image signals.

Preferably, the conversion unit converts the RGB video image signal intothe YUV video image signal having a YUV 4:2:0 format; and thecompression and encoding unit increases the bit rate of the compressionand encoding of the Y signal, up to a maximum of 1.5 times, inaccordance with the disconnection of the output paths of the U and Vvideo image signals.

In order to attain the aforementioned object, the present invention isalso directed to a control method for a moving picture real-timecommunications terminal, comprising: an input unit which inputs an RGBvideo image signal; a conversion unit which converts the RGB video imagesignal into a YUV video image signal including Y, U and V video imagesignals; a compression and encoding unit which digitally converts theYUV video image signal outputted from the conversion unit into movingpicture data, and compresses and encodes the moving picture data; acommunications unit which sends and receives the moving picture datacompressed and encoded by the compression and encoding unit, in realtime, to and from a communications terminal of another party connectedthrough a network; a bandwidth estimation unit which estimates change ina bandwidth of the network; and a switching circuit which connects anddisconnects output paths of the U and V video image signals from theconversion unit to the compression and encoding unit, the methodcomprising the steps of: disconnecting the output paths of the U and Vvideo image signals by operating the switching circuit, in response tothe change in the bandwidth estimated by the bandwidth estimation unit;and increasing a bit rate of compression and encoding of the Y signal bythe compression and encoding unit, in accordance with the disconnectingof the output paths of the U and V video image signals.

In order to attain the aforementioned object, the present invention isalso directed to a computer readable medium having embodied thereon acontrol program for performing a moving picture real-time communicationscontrol by a moving picture real-time communications terminalcomprising: an input unit which inputs an RGB video image signal; aconversion unit which converts the RGB video image signal into a YUVvideo image signal including Y, U and V video image signals; acompression and encoding unit which digitally converts the YUV videoimage signal outputted from the conversion unit into moving picturedata, and compresses and encodes the moving picture data; acommunications unit which sends and receives the moving picture datacompressed and encoded by the compression and encoding unit, in realtime, to and from a communications terminal of another party connectedthrough a network; a bandwidth estimation unit which estimates change ina bandwidth of the network; and a switching circuit which connects anddisconnects output paths of the U and V video image signals from theconversion unit to the compression and encoding unit, the controlprogram comprising: a first code segment for a step of disconnecting theoutput paths of the U and V video image signals by operating theswitching circuit, in response to the change in the bandwidth estimatedby the bandwidth estimation unit; and a second code segment for a stepof increasing a bit rate of compression and encoding of the Y signal bythe compression and encoding unit, in accordance with the disconnectingof the output paths of the U and V video image signals.

According to the present invention, the video image signal is switchedfrom color to black and white by disconnecting the output paths of the Uand V signals of the YUV video image signal, and the bit rate of thecompression and encoding of the Y signal is increased, by an maximumamount corresponding to the amount, of U and V data removed by thisdisconnection of the U and V signals. For example, if the conversionunit is converting the RGB video image signal into the YUV video imagesignal having the YUV 4:2:0 format, then by switching to a black andwhite image, the bit rate of the compression and encoding for the Ysignal alone is increased up to a maximum of 1.5 times. In other words,even if the bandwidth is reduced, by switching the video image signalthat is to be compressed and encoded to a black and white image signal,and then increasing the bit rate of the compression and encoding of theY signal, it is possible, conversely, to improve the image quality.Therefore, there is no reduction in the number of frames, or developmentof “jerkiness” in the movement of the image, in response to a reductionin the bandwidth, as in the related art.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature of this invention, as well as other objects and advantagesthereof, will be explained in the following with reference to theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures and wherein:

FIG. 1 is a block diagram of a real-time communications terminalaccording to an embodiment of the present invention;

FIG. 2 is a diagram showing a conceptual view of the transmission andreception of RTCP SR and RTCP RR packets, by first monitoring units andsecond monitoring units in the communication terminals; and

FIG. 3 is a flowchart showing the operations of the communicationterminal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a block diagram of a communications terminal 100 according toa preferred embodiment of the present invention. This communicationsterminal 100 is connected through a network 5 to the communicationsterminal 100 of another communicator, having a similar composition. Thecommunications terminal 100 converts a video image signal to a movingpicture signal digitally compressed and encoded and sends the movingpicture signal in the form of packets to the communications terminal 100of the other party. At the same time, the communications terminal 100receives, decodes and reproduces a moving picture that has beensimilarly digitally compressed and encoded, from the other party.

In order to simplify the description, the communications terminals 100are described as being connected in a one-to-one correspondence (namely,a unicast configuration), but a multicast or broadcast configuration canalso be possible. The connection path between the communicationsterminals 100 is specified, for example, by an exchange server (notshown) constituted by a SIP (Session Initiation Protocol) server, usinga network address (global IP (Internet Protocol) address, and the like),a port and an identifier (MAC (Media Access Control) address, or thelike). When a session has been established between the communicationsterminals 100 by the exchange server, then the communications terminals100 subsequently send and receive the data directly, without passingthrough the exchange server (i.e., in the peer-to-peer system).

The network 5 is constituted, for example, by a broadband network, suchas ADSL (Asymmetric Digital Subscriber Line), a fiber-optic network(FTTH (Fiber-To-The-Home)), or a cable television network, a narrow-bandnetwork, such as ISDN (Integrated Services Digital Network), or IEEE802.xx-compatible wireless communications, such as UWB (Ultra Wide Band)and Wi-Fi (Wireless Fidelity), or the like.

In the present embodiment, the network 5 is supposed to be a“best-effort” network, in which it cannot be guaranteed that a bandwidthof a prescribed value (communications speed) can be ensured at alltimes. In the network 5, the nominal maximum bandwidth is substantiallyrestricted by various factors, such as the distance between thetelephone exchange and the home, the communications speed between theADSL modems, increase and decrease in traffic, the communicationsenvironment of the other party of the session, and the like. In manycases, the effective value is a fraction of the nominal value. Thebandwidth of the network 5 is expressed in terms of bits per second(bps). For example, the nominal bandwidth of FTTH is generally 100 Mbps,but in actual practice, it may be limited to a few hundred kbps.

The communications terminal 100 comprises: a signal conversion unit 11,which converts an R, G; B video image signal inputted from a camera 10,or the like, into a video image signal comprising Y (luminosity signal),U (color difference signal Cb), and V (color difference signal Cr); abuffer 12, which temporarily accumulates the YUV video image signaltransferred to an encoding unit 13; the encoding unit 13, whichdigitally converts the YUV video image signal outputted from the buffer12 into moving picture data, and then compresses and encodes thisdigital data by means of an MPEG-4 method (MPEG stands for MovingPicture Experts Group), or the like; and a transmission unit 14, whichconverts the moving picture data compressed and encoded by the encodingunit 13, into packets, and transmits same to the communications terminal100 of another party.

Given the fact that the characteristics of the human eye are such thatit is sensitive to changes in luminosity but insensitive to changes incolor, then the conversion of the RGB signal into YUV video image signalby the signal conversion unit 11 is carried out in order to reduce theamount of color information while assigning a larger amount ofinformation to the luminance Y. This conversion is known as YUV 4:4:4,YUV 4:2:2, YUV 4:2:0, or the like, according to the sampling ratios ofthe Y, U and V signals.

YUV 4:4:4 means that the Y, U, V signals are converted at the sameratio. More specifically, an RGB signal which represents color by meansof a total of 8+8+8=24 bits, namely, 8 bits each for R, G and B, perpixel, is converted into coordinates on a YUV system, in which theluminosity Y, and color differences U and V, are acquired for all fourpixels of four pixels aligned in the horizontal direction. The number ofbits per pixel of YUV signal after conversion to a YUV 4:4:4 format is8+8+8=24 bits (compression ratio 1), and hence the amount of data perpixel does not change. The YUV 4:4:4 format is used for special fields,such as medical use, movies, and the like.

In the YUV 4:2:2 format, the data volumes of the color differencesignals U and V constituting a pixel are respectively thinned to ½ inthe horizontal direction. In other words, the luminance Y is acquiredfor all of the four pixels in the horizontal direction, but the colordifference signals U and V are only acquired every two pixels.Therefore, the number of bits per pixel of the YUV signal afterconversion to the YUV 4:2:2 format, is 8+4+4=16 bits (namely, acompression rate of ⅔). The YUV 4:2:2 format is used generally intelevision studios, and other professional uses.

In the YUV 4:2:0 format, the color difference signals U and V in the YUV4:2:2 format are thinned to ½ in the vertical direction. Therefore, thenumber of bits per pixel of the YUV signal after conversion to the YUV4:2:0 format, is 8+2+2=12 bits (namely, a compression rate of ½). TheYUV 4:2:0 format is commonly used in encoders using the MPEG-4method, orthe like, and it reduces the data volume by ½ per pixel.

Whatever the format of the YUV video image signal converted by thesignal conversion unit 11, either YUV 4:4:4 , YUV 4:2:2 , or YUV 4:2:0 ,the same theory is established, but the most beneficial effects areobtained in the case of the YUV 4:4:4 format or the YUV 4:2:2 format.Hereinafter, unless specified otherwise, the following descriptionsrelate to the YUV 4:2:0 format.

The data volume of the moving picture data compressed and encoded by theencoding unit 13 is also dependent on the noise in the video imagesignal outputted from the camera 10, the resolution of the video imagesignal, and the number of frames per second (frame rate), and the like.

The compressed data volume (number of packets) generated by the MPEG-4or other type of encoder is principally dependent on the following imageparameters:

-   -   the qualities of the image capture device: camera        characteristics, captured image qualities (noise), and the like;    -   the image resolution: number of pixels in the vertical and        lateral directions;    -   frame rate: number of frames per second;    -   GOP (group of pictures) length: length (number of frames)        between two intra frames; and    -   estimated bit rate : estimated current bandwidth of        communications path (this temporally varies, since it is a best        effort communications path).

Here, the object of the present embodiment is considered with respect toa case where it is sought to transmit the movement and shape of anobject more accurately, rather than the color information. For example,considering an application in which persons with hearing difficultiesperform video communications by using sign language, then the importantelement is the sign language, in other words, manual gestures and facialexpression. In this case, it is desirable to depict the movementclearly, even in the form of black and white images, rather thandepicting degraded color images.

If the bandwidth bit rate has declined, then in the case of a standardvideo telephone, or the like, a common method for reducing the datavolume is to reduce the frame rate. For example, the rate becomes 15frames per second. However, 15 frames per second is not sufficient todisplay smooth manual gestures, and a frame rate of 30 frames persecond, similar to television, is desirable.

The packet transmission protocol of the transmission unit 14 uses RTP(Real-Time Transport Protocol), which is a UDP (User Datagram Protocol)type commonly used in the transmission of streaming media, and the like.RTP comprises an RTP header, which is a generic header for conveyingreal-time information, and an RTP payload, which includes the actualmoving picture data. The RTP payload is specified by the RFC (RequestFor Comments) for each data format, such as MPEG-1, MPEG-2, MPEG-4,H264, H264/MPEG-4 AVC, and so on, but the present method is notdependent on the compression method used for the digital movingpictures.

In UDP, in contrast to TCP (Transmission Control Protocol), packet losscountermeasures and retransmission control, and other such operations,are not carried out. Therefore, it is used in conjunction with acommunications status report based on RTCP (RTP Control Protocol). Thisreport is obtained by a first monitoring unit 15.

More specifically, with reference to FIG. 2, the first monitoring unit15-A provided in the communications terminal 100-A (one of thecommunications terminals 100) sends an SR (Sender Report) type RTCPpacket (RTCP SR) (a monitoring packet) to the communications terminal100-B (the other of the communications terminals 100) of the other partyto the session, at regular intervals (namely, every time a prescribedtime period has passed). The first monitoring unit 15-A stores a timestamp indicating the transmission timing of the RTCP SR packets, and thenumber of transmitted packets, which is the total number of RTCP SRpackets sent within a fixed time period.

The communications terminal 100 comprises a second monitoring unit 25.When RTCP SR packets are received from the communications terminal100-A, the second monitoring unit 25-B provided in the communicationsterminal 100-B calculates the number of lost RTCP SR packets received,by finding the sequence numbers of the sequence number fields in theheaders of the RTCP SR packets. The second monitoring unit 25-B sends anRR (Receiver Report) type RTCP packet (RTCP RR) stating the number oflost packets, to the communications terminal 100-A. The RTCP RR packetalso states the time period from the reception of the RTCP SR packetuntil the transmission of the RTCP RR packet (for the sake ofconvenience, this time period is called the “response time”).

Upon receiving the RTCP RR from the second monitoring unit 25-B, thefirst monitoring unit 15-A calculates the RTT (Round Trip Time), whichis the time period from the transmission time of the RTCP SR packetuntil the reception time of the RTCP RR packet, minus the response time.Furthermore, the first monitoring unit 15-A refers to the number of RTCPSR packets transmitted, and the number of lost packets in the RTCP RR,and it calculates the packet loss rate within the prescribed timeperiod, which is equal to “(number of lost packets)/(number oftransmitted packets)”. The first monitoring unit 15-A sends the RTT andthe packet loss rate, in the form of a communications status report, tothe bandwidth estimation unit 16-A.

It is thought that a suitable interval at which to issue the monitoringpackets is ten seconds to several tens of seconds, but if estimated bymeans of a single trial of a monitoring packet, it is often impossibleto ascertain the network status accurately, and therefore, estimationaccuracy is improved if the issuance of monitoring packets is dividedinto a plurality of operations, and an estimate is made by finding theaverage value thereof. If the number of monitoring packets becomeslarge, then this itself becomes a cause of narrowing of the bandwidth,and therefore, desirably, the monitoring packets occupy 2% to 3% of thetotal communications data volume.

Apart from the method described above, it is also possible to obtain acommunications status report by using various QoS (Quality of Service)control technologies for the bandwidth estimation unit 16.

Furthermore, the communications terminal 100 comprises: a reception unit21, which receives compressed and encoded packets of moving picture datasent in packets by the communications terminals 100 of the other party;a decoding unit 22, which decodes and reproduces the received videoimage signal into the YUV video image signal; a buffer 23, whichaccumulates the YUV video image signal reproduced by the decoding unit22; and a display device 24, which reproduces and displays a video imagefrom the YUV video image signal outputted from the buffer 23.

If the transmission frame rate is reduced when it is estimated thatthere has been a reduction in the transmission bandwidth of the network5, as in the related art, in other words, if the frames are thinned out,then the movement of the object in the video image becomes rough andjerky, rather than smooth. This problem, which occurs frequently inreal-time video image transmission, is a particular issue in cases whereit is sought to transmit the movement and shape of the objectaccurately, rather than the color information. For example, if personswith hearing difficulties are performing sign language, then if they arenot able to clearly grasp each other's manual gestures and facialmovements, this will inevitably present a major obstacle tocommunication. Consequently, in the communications terminal 100 of thepresent embodiment, priority is given to switching the moving picturedata that is to be transmitted, from a color image to a black and whiteimage, rather than thinning out the frames or reducing the overall imagequality by simply reducing the compression bit rate for the whole image,in response to an estimated reduction in the transmission bandwidth ofthe network 5.

In other words, the communications, terminal 100 comprises: a switchingcircuit 17, which connects and disconnects the output paths of the U andV video image signals from the buffer 12 to the encoding unit 13; and aswitching control unit 30, which controls the operation of the switchingcircuit 17. The switching control unit 30 activates the switchingcircuit 17 and disconnects the output paths of the U and V video imagesignals, as and when the bandwidth estimation unit 16 estimates thatthere has been a reduction in the transmission bandwidth. If the outputpaths of the U and V video image signals are disconnected, then thevideo image signal to be encoded includes the Y signal only, and hencethe video image signal changes from color to black and white.

Moreover, in accordance with the disconnection of the output paths ofthe U and V video image signals, the encoding unit 13 increases theencoding bit rate of the Y signal, taking as an upper limit for thisincrease the data volume removed by the disconnection of the outputpaths of the U and V video image signals. More specifically, in the caseof the YUV 4:2:0 format, when the output paths of the U and V videoimage signals are disconnected, 4 bits of 12 bits per pixel are removed,in other words, there is approximately a 33% reduction in the datavolume. In the case of the YUV 4:2:2 format, 8 bits of data are removedin every 16 bits per pixel, which corresponds a 50% data reduction rate.In the case of the YUV 4:4:4 format, 16 bits of data are removed inevery 24 bits per pixel, which corresponds a 67% data reduction rate.Therefore, it is possible to increase the encoding bit rate of the Ysignal by a maximum of 1.5 times, in the case of the YUV 4:2:0 format, amaximum of 2.0 times in the case of the YUV 4:2:2 format, and a maximumof 3.0 times in the YUV 4:4:4 format.

FIG. 3 is a flowchart showing the above-described operation. If thebandwidth estimation unit 16 estimates that the transmission bandwidthhas reduced (“YES” at S1), then the switching control unit 30 activatesthe switching circuit 17 and disconnects the output paths of the U and Vvideo image signals (S2). The encoding unit 13 increases the encodingbit rate for the Y signal, using as an upper limit for this increasecorresponding to the data volume removed by disconnecting the outputpaths of the U and V video image signals (S3). This sequence ofoperation in steps S1 to S3 are repeated according to requirements.

In the case of the YUV 4:2:0 format, for example, the encoding bit rateof the Y signal is increased by an maximum of 1.5 times (=12 bits/8bits), which is the ratio of the data volume per pixel saved by thedisconnection of the U and V signals. Although the video image changesto a black and white image, the image quality of the moving picture isimproved since the bit encoding rate of the Y, signal is increased. Inother words, even if the bandwidth is 400 kbps, for example, byswitching to a black and white image, it becomes theoretically possibleto encode a moving picture at a maximum bit rate of 600 kbps, and thuseven if the bandwidth declines, it is still possible to transmit amoving picture having 50% improved quality. Since the frame rate is notreduced in response to a reduction in the bandwidth as in the relatedart, the image does not become rough and jerky when the bandwidthreduces.

As and when the bandwidth estimation unit 16 estimates that thetransmission bandwidth has increased, the switching control unit 30activates the switching circuit 17 and reconnects the output paths ofthe U and V video image signals, and the encoding bit rate of the YUVsignal is returned to that of the original color moving picture signal.In other words, if there is a reduction in the bandwidth, then the imageis switched to a black and white image, and if the bandwidth improvesagain, then a color video image is transmitted at the original encodingbit rate.

The point in the bandwidth at which to switch the bit rate is setaccording to each individual system, but judging on the basis of currentcompression technologies and conditions in the Internet, the thresholdvalue of the bandwidth is preferably between 300 kbps and 500 kbps. Inthe present embodiment, it is set to 400 kbps.

Furthermore, a method and a program for implementing the steps S1 to S3described above in the communications terminal 100 are also included inthe present invention. Moreover, it is also possible to realize theswitching circuit, and the like, described in the present embodiment, bymeans of control implemented by a program.

It should be understood, however, that there is no intention to limitthe invention to the specific forms disclosed, but on the contrary, theinvention is to cover all modifications, alternate constructions andequivalents falling within the spirit and scope of the invention asexpressed in the appended claims.

1. A moving picture real-time communications terminal, comprising: aninput unit which inputs an RGB video image signal; a conversion unitwhich converts the RGB video image signal into a YUV video image signalincluding Y, U and V video image signals; a compression and encodingunit which digitally converts the YUV video image signal outputted fromthe conversion unit into moving picture data, and compresses and encodesthe moving picture data; a communications unit which sends and receivesthe moving picture data compressed and encoded by the compression andencoding unit, in real time, to and from a communications terminal ofanother party connected through a network; a bandwidth estimation unitwhich estimates change in a bandwidth of the network; a switchingcircuit which connects and disconnects output paths of the U and V videoimage signals from the conversion unit to the compression and encodingunit; and a switching control unit which disconnects the output paths ofthe U and V video image signals by operating the switching circuit, inresponse to the change in the bandwidth estimated by the bandwidthestimation unit, wherein the compression and encoding unit increases abit rate of compression and encoding of the Y signal, in accordance withdisconnection of the output paths of the U and V video image signals. 2.The moving picture real-time communications terminal as defined in claim1, wherein: the conversion unit converts the RGB video image signal intothe YUV video image signal having a YUV 4:2:0 format; and thecompression and encoding unit increases the bit rate of the compressionand encoding of the Y signal, up to a maximum of 1.5 times, inaccordance with the disconnection of the output paths of the U and Vvideo image signals.
 3. A control method for a moving picture real-timecommunications terminal, comprising: an input unit which inputs an RGBvideo image signal; a conversion unit which converts the RGB video imagesignal into a YUV video image signal including Y, U and V video imagesignals; a compression and encoding unit which digitally converts theYUV video image signal outputted from the conversion unit into movingpicture data, and compresses and encodes the moving picture data; acommunications unit which sends and receives the moving picture datacompressed and encoded by the compression and encoding unit, in realtime, to and from a communications terminal of another party connectedthrough a network; a bandwidth estimation unit which estimates change ina bandwidth of the network; and a switching circuit which connects anddisconnects output paths of the U and V video image signals from theconversion unit to the compression and encoding unit, the methodcomprising the steps of: disconnecting the output paths of the U and Vvideo image signals by operating the switching circuit, in response tothe change in the bandwidth estimated by the bandwidth estimation unit;and increasing a bit rate of compression and encoding of the Y signal bythe compression and encoding unit, in accordance with the disconnectingof the output paths of the U and V video image signals.
 4. A computerreadable medium having embodied thereon a control program for performinga moving picture real-time communications control by a moving picturereal-time communications terminal comprising: an input unit which inputsan RGB video image signal; a conversion unit which converts the RGBvideo image signal into a YUV video image signal including Y, U and Vvideo image signals; a compression and encoding unit which digitallyconverts the YUV video image signal outputted from the conversion unitinto moving picture data, and compresses and encodes the moving picturedata; a communications unit which sends and receives the moving picturedata compressed and encoded by the compression and encoding unit, inreal time, to and from a communications terminal of another partyconnected through a network; a bandwidth estimation unit which estimateschange in a bandwidth of the network; and a switching circuit whichconnects and disconnects output paths of the U and V video image signalsfrom the conversion unit to the compression and encoding unit, thecontrol program comprising: a first code segment for a step ofdisconnecting the output paths of the U and V video image signals byoperating the switching circuit, in response to the change in thebandwidth estimated by the bandwidth estimation unit; and a second codesegment for a step of increasing a bit rate of compression and encodingof the Y signal by the compression and encoding unit, in accordance withthe disconnecting of the output paths of the U and V video imagesignals.